Reverse voltage protection for motor drive

ABSTRACT

A reverse voltage protector for a motor drive is presented. The circuit is in series with an external electrical power source and the motor drive or electrical motor. The reverse voltage protector comprises a switch having a zener diode to protect the switch from overvoltage and a resistor regulating electrical current flow in the control terminal of the switch.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This patent application is a continuation-in-part of copending U.S. patent application Ser. No. 09/795,045, filed Feb. 26, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates generally to motor drives for electric motors, and more particularly relates to an apparatus for reverse voltage protection of motor drives.

BACKGROUND OF THE INVENTION

[0003] A schematic drawing of a prior art motor drive 10 for an electrical motor 12 (the ProAct Generation 1, which is a form of Limit Angle Torque motor (LAT) commercially available from Woodward Governor) is illustrated in FIG. 1. The motor drive 10 serves the purposes of conditioning the electrical power received from an external power source 14 and maintaining the proper electrical power levels for driving the electrical motor 12. The motor drive 10 includes an input diode 16 for ensuring reverse voltage protection, an electromagnetic (EMI) filter 17 for filtering out high frequency interference, and a bus capacitor 18 for storing electrical power and smoothing out any spikes or intermittent declines in the electrical power and a switch network for modulating electrical energy to the motor.

[0004] In the prior art circuit illustrated in FIG. 1, it should first be noted that the motor drive 10 is not integrated with electrical motor 12 but instead is intentionally mounted remotely such that the motor drive 10 is subject to relatively low temperatures of about a maximum of 85° Celsius. The motor 12 is driven by an H-bridge configuration comprising four switches 20, 21, 22, 24. When the first and fourth switches 20, 23 are closed (with switches 21, 22 open), the motor 12 is driven in a first rotational direction. When the second and third switches 21, 22 are closed (with switches 20, 23 open), the motor 12 is driven in a second rotational direction.

[0005] When the external power source 14 for the motor drive 10 is a DC voltage power bus, it is possible that the motor drive 10 could be hooked up backwards (i.e., reverse-biased) to the external power source 14. Many components in the motor drive 10 will be destroyed if they are hooked up backwards to a power source. Other components of the motor drive 10 could behave unpredictably after being reverse biased, which could result in an unsafe response from the motor 12.

[0006] In prior art designs, the input diode 16 is used to prevent destruction of the motor drive in the event that the battery is installed the opposite way. The use of diode 16 introduces problems in the motor drive 10. The input current of the motor drive 10 produces a forward voltage drop in the diode, resulting in power being dissipated in the diode 16. For high input currents, the dissipated power can be significant, requiring heat sinks or other cooling means to keep the diode temperature below the diode's maximum allowable junction temperature. The heat sink or other cooling means increases package size and cost. Another problem the diode 16 creates is when the motor 12 acts as a generator during transient conditions, resulting in inductive flyback current flowing into the motor drive 10. This forces the motor drive 10 to be designed to handle the inductive flyback. One way the prior art has dealt with this specific problem has been to incorporate a very large internal bus capacitance to handle and temporarily store this inductive flyback current load. The use of the very large internal bus capacitance results in an increased size and cost of the motor drive 10. Another method to deal with the inductive flyback is to use an active snubber as described in U.S. patent application Ser. No. 09/795,045, filed Feb. 26, 2001, to clamp the voltage across the bus capacitor 18.

[0007] Industry improved the efficiency of the reverse voltage protection by adding a switch in parallel with the input diode. In the prior art circuit illustrated in FIG. 2a, a metal oxide semiconductor field effect transistor (i.e., MOSFET) 30 is used. Those persons skilled in the art recognize that the MOSFET 30 has an inherent body diode 31 (see FIG. 2b) that can be used in place of the input diode 16 of FIG. 1. The driver power supply 32 generates a voltage at the MOSFET gate sufficiently higher than the input voltage that results in a voltage across the gate to source that turns the MOSFET 30 on. In operation, the circuit of FIG. 2 works as follows. Initially, there is no voltage potential across the gate to source of the MOSFET 30 because the driver power supply 32 is off, resulting in the MOSFET 30 being off. When a properly polarized input voltage is applied to the input of the motor drive 10, the body diode 31 conducts current that charges up the capacitor 18 and the driver power supply 32. The driver power supply 32 starts to operate and outputs a voltage to the gate of the MOSFET 30 that is approximately 9 to 15 volts higher than the input voltage. This turns the MOSFET 30 on and input current flows through the MOSFET channel rather than the body diode. Once on, the MOSFET 30 conducts current in both directions, allowing motor inductive flybacks to be absorbed by the external power source 14. If a reverse input voltage is applied, the body diode 31 blocks current flow and the MOSFET 30 will remain off because there is no gate to source voltage. The disadvantage of FIG. 2 is that the driver power supply 32 is needed to provide the gate to source voltage. The driver power supply 32 typically requires a boost switching power supply or a charge pump power supply circuit. These circuits add cost, size, weight, and complexity to the motor drive design.

BRIEF SUMMARY OF THE INVENTION

[0008] In light of the above, it is an objective of the present invention to provide a motor drive for an electrical motor that increases the efficiency of the reverse voltage protection.

[0009] In that regard, it is also an object of the present invention to achieve the foregoing object without creating cost or size drawbacks.

[0010] In accordance with these and other objectives, the present invention is directed toward a motor drive with a passively biased reverse voltage protection circuit. The reverse voltage protection circuit comprises a switch in parallel with a diode in one of the input lines to the motor drive. When a properly polarized input voltage is applied to the input of the motor drive, the switch turns on, allowing input power to flow to the motor drive. If a reverse input voltage (i.e., improperly polarized voltage) is applied, the switch remains off and the diode blocks current flow.

[0011] Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:

[0013]FIG. 1 is a schematic illustration of a prior art reverse voltage protection circuit for a motor drive.

[0014]FIG. 2 is a schematic illustration of a prior art reverse voltage protection circuit for a motor drive.

[0015]FIG. 3 is a schematic illustration of a reverse voltage protection circuit for a motor drive according to a first embodiment of the present invention.

[0016]FIG. 4 is a schematic illustration of a reverse voltage protection circuit for a motor drive according to a second embodiment of the present invention.

[0017] While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0018] For purposes of illustration, a first embodiment of the present invention has been illustrated in FIG. 3 as a electrical motor unit 40 comprising an electrical motor 42 and motor drive 44. The unit 40 can be electrically connected to an external or integrated power source 50 (shown herein as a battery for engine applications although non-battery sources are commonly used in turbine applications) for receipt of electrical power to drive the motor 42. Electrical motors of this type typically operate with a power source 50 that is between 8 and 32 volts and between 70 and 400 watts. The unit 40 is typically sold and mounted as a single component without the need to mount the electronics assembly (including motor drive 44) separate or remote from the electrical motor 42. Further structural details and advantages of the unit 40 are described in U.S. patent applications Ser. Nos. 09/793151, 09/793225, 09/793356, and 09/795,045, all filed on Feb. 26, 2001, and owned by the present assignee, the entire disclosures of which are hereby incorporated by reference.

[0019] The motor drive 44 is interposed on the bus 60 running from the electrical power source 50 to the electrical motor 42. The motor drive 44 an electromagnetic (EMI) filter 62 for filtering out high frequency interference on the bus 58, a bus capacitor 46 for smoothing out voltage spikes and natural inconsistencies in the electrical power flow to the motor 42, and a switch network (comprised of switches 52-58) for modulating electrical energy to the motor.

[0020] In the embodiment shown, the electrical motor 42 is operatively arranged in an H-bridge circuit comprising four switches 52-58 as an exemplary form of a switch network. When the first and fourth switches 52, 58 are closed (with switches 54, 56 open) in a first state, the motor 42 is driven in a first rotational direction. When the second and third switches 54, 56 are closed (with switches 52, 58 open) in a second, the motor 42 is driven in a second rotational direction. A third “free wheel” state is also provided in which switches 54 and 58 are closed. Torque is proportional to current. The current is modulated by the ratio of switch states (e.g. switching between first and third states to effect a selected torque in a first angular direction and switching between second and third states to effect a selected torque in a second angular direction).

[0021] The reverse voltage protection of the present invention utilizes a switch 70 having an integral body diode arranged in series with the battery 50 and the motor 42 (or the switch network). In the embodiment shown, the switch 70 is an n-channel MOSFET. When the proper polarity of input voltage is applied to the motor drive 44, the integral body diode of the switch 70 conducts current, allowing a positive potential to develop across the gate 72 (i.e., control terminal) relative to the source 74 (i.e., output terminal). When the potential difference between the gate 72 and source 74 reaches approximately 10 volts, the switch 70 turns on. All current will flow (in the reverse mode of operation) through the drain 73 (i.e., input terminal) and source 74 provided that the voltage across the switch 70 does not exceed the forward voltage drop of the integral body diode (typically 0.6 volts for silicon devices). This is ensured if the peak drain current multiplied by the on resistance of the switch 70 is less than the integral body diode forward voltage drop. The impedance of the switch 70 is lower than the prior art diode, which reduces power dissipation in the motor drive 44. The zener diode 76 is used to protect the gate 72 from voltages higher than the switch's maximum gate voltage. The resistor 78 is used to limit the current flow and power dissipation in the zener diode. In the event the terminals of the battery 50 are reversed due to improper installation, the negative voltage on the gate 72 will not allow the switch 70 to activate and therefore the switch 70 remains open preventing a completed circuit and the integral body diode will block current flow.

[0022] Turning now to FIG. 4, an alternate embodiment of the present invention is shown. In this embodiment, the switch is located on the positive input terminal of the motor drive 44. The switch 88 is a p-channel device such as a p-channel MOSFET or IGBT. The operation of the reverse voltage protection is similar to the n-channel switch of FIG. 3. When the proper polarity of input voltage is applied to the motor drive 44, the integral body diode of the switch 88 conducts current, allowing a negative potential to develop across the gate 90 relative to the source 92. When the potential difference between the gate 90 and source 92 reaches a magnitude of approximately 10 volts, the switch 88 turns on and current flows through the drain 91 and the source 92 in the reverse mode. The zener diode 76 is used to protect the gate 90 from voltages higher than the switch's maximum gate voltage. The resistor 78 is used to limit the current flow and power dissipation in the zener diode. In the event the terminals of the battery 50 are reversed due to improper installation, the positive voltage on the gate 90 will not allow the switch 88 to activate and therefore the switch 88 remains open preventing a completed circuit and the integral body diode will block current flow.

[0023] In certain applications, the power source may also be unable to absorb the energy because of its internal make up, or it may be unable to absorb the energy effectively because of the attenuation of the EMI filter 62 and/or parasitic line impedance. In those instances, the active snubber circuit 80 comprising switch 82, resistor 84, and voltage sensor/drive 86 protects the motor driver from over voltage conditions if the power lines were opened while significant current was following in the motor. Further details on the snubber circuit 80 are in U.S. patent application Ser. No. 09/795,045, filed Feb. 26, 2001, hereby incorporated in its entirety by reference.

[0024] The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. For example, those skilled in the art will recognize that a three phase motor and motor drive configuration (e.g., a pole-pair for each motor phase) can be used. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. 

What is claimed is:
 1. A reverse voltage protector interposed between an electrical power source having a positive terminal and a negative terminal and an electrical motor, the reverse voltage protector comprising an n-channel switch having a gate terminal, a drain terminal, and a source terminal, the gate terminal connected to the positive terminal, the source terminal connected to the electrical motor, the drain terminal connected to the negative terminal, the switch remaining passively open and preventing flow of electrical power when the positive terminal and negative terminal are connected to the motor in reverse, the switch allowing flow of electrical power when the positive terminal and negative terminal are correctly connected to the motor.
 2. The reverse voltage protector of claim 1 such that current flows in the switch from the source terminal to the drain terminal when the switch is on.
 3. The reverse voltage protector of claim 1 further comprising: a zener diode arranged in parallel with the gate terminal and the source terminal; and a resistor connected to the gate terminal and the positive terminal.
 4. The reverse voltage protector of claim 1 wherein the switch is a metal oxide semiconducter field effect transistor.
 5. The reverse voltage protector of claim 1 further comprising a diode in parallel with the switch.
 6. A reverse voltage protector interposed between an electrical power source having a positive terminal and a negative terminal and an electrical motor, the reverse voltage protector comprising: a p-channel switch having a gate terminal, a drain terminal, and a source terminal, the gate terminal connected to the negative terminal, the source terminal connected to the electrical motor, the drain terminal connected to the positive terminal, the switch remaining passively open and preventing flow of electrical power when the positive terminal and negative terminal are connected to the motor in reverse, the switch allowing flow of electrical power when the positive terminal and negative terminal are correctly connected to the motor;
 7. The reverse voltage protector of claim 6 wherein the switch is a metal oxide semiconducter field effect transistor.
 8. The reverse voltage protector of claim further comprising: a zener diode arranged in parallel with the gate terminal and the source terminal; and a resistor connected to the gate terminal and the positive terminal.
 9. The reverse voltage protector of claim 8 wherein the gate terminal has a maximum rated gate voltage and the zener diode is sized so that a maximum voltage across the gate terminal and source terminal is below the maximum rated gate voltage.
 10. A reverse voltage protector interposed between an electrical power source having a positive terminal and a negative terminal and an electrical motor, the reverse voltage protector comprising: a switch having a control terminal, an input terminal, an output terminal, and a diode between the input terminal and the output terminal, the output terminal connected to the electrical motor, the control terminal connected to one of the positive terminal and the negative terminal and the input terminal connected to an other of the positive terminal and the negative terminal such that the switch turns on when the positive terminal and the negative terminal are correctly connected, the switch remaining passively open when the positive terminal and the negative terminal are connected to the motor in a reverse orientation.
 11. The reverse voltage protector of claim 10 further comprising: a zener diode arranged in parallel with the control terminal and the output terminal; and a resistor connected to the control terminal and the one of the positive terminal and the negative terminal.
 12. The reverse voltage protector of claim 10 wherein current flows through the switch in a reverse direction, the reverse direction being from the output terminal to the input terminal, the diode has a forward voltage drop, the switch has an on resistance, and the switch is selected so that the on resistance does not exceed a ratio of the forward voltage drop divided by a predetermined current magnitude flowing through the switch.
 13. The reverse voltage protector of claim 12 wherein the predetermined current magnitude is a magnitude of a maximum continuous current flowing through the switch.
 14. The reverse voltage protector of claim 8 wherein the control terminal has a maximum rated voltage and the zener diode is sized so that a maximum voltage across the control terminal and output terminal is below the maximum rated voltage.
 15. The reverse voltage protector of claim 8 wherein the resistor has an impedance and the impedance is selected so power dissipation in the zener diode is limited to a predetermined value. 